1. Field of the Invention
The present invention relates to reducing effects of digital-circuit switching on analog circuitry located within the same integrated circuit (IC) and, more specifically, to reducing effects of local reference-potential fluctuations induced by digital-circuit switching.
2. Description of the Related Art
Mixed-signal ICs have both analog and digital circuits formed on a common substrate. For example, a mixed-signal IC may contain an analog circuit, such as an operational amplifier, a digital-to-analog converter (DAC), and/or an analog-to-digital converter (ADC), as well as a digital circuit, such as a digital signal processor (DSP). A representative mixed-signal application might involve (i) converting an analog input signal into a corresponding digital signal using an ADC, (ii) processing the data using a DSP, and (iii) converting the processed data into an analog output signal using a DAC.
Various forms of noise can be communicated from digital to analog circuits within a mixed-signal IC via the common substrate. One form of such noise (often referred to as digital switching noise) is caused by switching ON and OFF the mixed-signal IC's digital circuit or a portion thereof. Digital circuits, for example, generate high frequencies and harmonics and other noise due, in part, to the sharp edges of the digital waveforms used for clock signals and the like. This digital circuit noise can be communicated to sensitive analog circuit sections in the IC through the common substrate and can adversely affect their operation because analog signals are typically referenced to the local ground potential, and not to the external one. For example, in a representative mixed-signal IC, for a digital current of about 80 mA, the digital substrate might become biased with respect to the external ground potential by about 4 mV. Due to low substrate resistivity, voltage shifts induced in the digital substrate are coupled into the analog substrate. As a result, analog signals in the IC can shift by about 4 mV each time the IC's digital circuits are turned ON. For a 13-bit DAC with a least-significant-bit (LSB) voltage step of about 0.5 mV, a 4-mV ground-potential bias amounts to an error of about 8 LSBs, which is a rather significant amount.